In the related art, most optical receivers and optical communications involve continuous mode communication. One new method for optical communication involves burst mode communication. Burst mode communication is useful, for example, in point to multipoint communication, such as occurs when a single operator is linked to many users. In such operation mode, many users are connected to a single operator using fiber optic lines that are split between the users. In order to prevent interference between the users, every user performs the transmission using a different carrier. Thus, at multiple times, the user communication is quiet, then the user starts a burst of transmission, and then the user shuts down again, waiting for a next period for transmission.
The difficulties of receiving and distinguishing between receivers are exacerbated by typically large variations in the magnitude of power of transmission bursts between different users. FIG. 1 is a graphical representation of a signal 100 demonstrating a low power burst following a high power burst, that typically occurs in a burst communication, and as is known in the art. The power difference between two successive bursts in signal 100 can be about 15-25 dB. Also, the high power burst can raise the average power level of a successive low power burst, which decays slowly over time.
Architecture of the prior art for continuous mode optical receivers fail to operate properly for burst mode communications. FIG. 2 shows a typical architecture of a continuous mode optical receiver 200. The receiver 200 includes an optical detector (e.g., a photodiode) 210 coupled to an input of a transimpedence amplifier (TIA) 220, a limiter-amplifier 230, and a direct current (DC) restoration loop 240. The TIA stage 220 receives a weak signal output from the optical detector 210 coupled, for example, to an optical fiber line, and amplifies the signal. The limiter-amplifier 230 clips the output signal of the TIA stage 220 at specific high and low voltage levels.
The optical receiver 200 must discriminate between a high level and a low level signal that is received. The optical signal typically includes “on/off keying,” which consists of transmitted “on” signals (also referred to as “ones”) and “off” signals (also referred to as “zeros”). With an optical device, the transmitted “on” signal is a pulse of light, while the transmitted “off” signal is the non-transmittal of light. For this purpose, the current produced by the received light is amplified by the TIA 220 and the DC restoration loop 240 filters the noise from the amplified signal, i.e., removes the DC portion of the signal. Examples of circuits implementing DC restoration loops may be found in U.S. Pat. Nos. 6,876,259, 6,720,827, and 6,552,605, each of which is incorporated herein by reference for their useful background descriptions of the state of the art heretofore.
Since the data is transmitted in bursts, a problem arises in that the optical receiver 200 must receive and distinguish bursts of data. The receiver 200 must recognize each transmitter that transmits data, and the receiver typically must estimate the power of the data to distinguish among bursts. In order to make this determination, the receiver must acquire the signal for the data burst within a short time period at the beginning of the burst.
In the prior art, continuous mode transmission and reception has typically been used with two station transmitters, from which data is continuously transmitted. That is, no stopping and restarting of data occurs, as is the case with burst mode transmission. As a result, in continuous transmission, it has not mattered how long it takes for the receiver to acquire the signal, and thus the receiver is not designed to acquire signal in a short period of time as required in burst transmission. Furthermore, trying to perform DC restoration on burst signals, ends with the inability to differentiate between light and dark, i.e., between “ones” and “zeros”. FIG. 1B shows an exemplary output signal 110 produced by the optical receiver 200 in response to the signal 100. In such signal a RX threshold that is typically utilized for distinguishing between “ones” and “zeros” cannot be properly set.
Therefore, it would be advantageous to provide a TIA circuit usable for burst mode communications.